Reconnecting a wind power plant to a utility grid

Abstract

It is described a method for coupling plural wind turbines to a utility grid after the wind turbines have been disconnected from the utility grid, the method comprising: closing a main breaker arranged between the utility grid and a main transformer resulting in reactive current flowing from the utility grid to the main transformer; closing a first feeder switch arranged between the main transformer and a first group of wind turbines of the plural wind turbines; selecting a first reference voltage; operating first converters of the first group of wind turbines in a voltage control mode according to the first reference voltage; and closing a second feeder switch.

The invention claimed is: 
     
       1. Method for coupling plural wind turbines to a utility grid after the wind turbines have been disconnected from the utility grid, the method comprising:
 closing a main breaker arranged between the utility grid and a main transformer resulting in reactive current flowing from the utility grid to the main transformer; 
 closing a first feeder switch arranged between the main transformer and a first group of wind turbines of the plural wind turbines resulting in reactive current flowing from the main transformer to first wind turbine transformers of the first group of wind turbines; 
 selecting a first reference voltage based on an expected voltage drop that occurs when a second feeder switch arranged between the main transformer and a second group of wind turbines of the plural wind turbines is closed, the first group comprising a smaller number of wind turbines than the second group of wind turbines; 
 operating first converters of the first group of wind turbines in a voltage control mode according to the first reference voltage; and 
 closing the second feeder switch. 
 
     
     
       2. Method according to  claim 1 , wherein the first converters are operated for a time period in order to stabilize a voltage at a point of interconnection between the utility grid and the main breaker. 
     
     
       3. Method according to  claim 2 , wherein the first reference voltage is larger than a nominal reference voltage at the point of interconnection. 
     
     
       4. Method according to  claim 3 , wherein the first reference voltage is the nominal reference voltage plus half of an expected voltage drop that occurs at the point of interconnection when the second feeder switch is closed. 
     
     
       5. Method according to  claim 1 , wherein the first wind turbine converters supply reactive power to the second wind turbine transformers. 
     
     
       6. Method according to  claim 1 , further comprising:
 closing a feeder switch between the main transformer and the a dynamic VAR controller before closing the first feeder switch; and 
 operating the dynamic VAR controller according to a SVC reference voltage that is selected based on an expected voltage drop that occurs at the point of interconnection when the first feeder switch is closed. 
 
     
     
       7. Method according to  claim 1 , further comprising:
 selecting a second reference voltage based on an expected voltage drop that occurs when a third feeder switch arranged between the main transformer and a third group of wind turbines of the plural wind turbines is closed; 
 operating second converters of the second group of wind turbines, and in particular also the first converters, in a voltage control mode according to second reference voltage; and 
 closing the third feeder switch. 
 
     
     
       8. Method according to  claim 7 , repeated for any feeder switch, until all wind turbines intended to be connected to the main transformer are connected to the main transformer. 
     
     
       9. Method according to  claim 8 , further comprising:
 operating all converters of all wind turbines connected to the main transformer according to the nominal reference voltage in voltage control mode. 
 
     
     
       10. Method according to  claim 8 , further comprising, if wind power plant comprises a further main transformer intended to be connected with further groups of wind turbines:
 parallel or semi parallel or sequentially closing a further main breaker arranged between the utility grid and the further main transformer resulting in reactive current flowing from the utility grid to the further main transformer; 
 operating all converters of wind turbines connected to the main transformer according to a further first reference voltage, different from the nominal reference voltage, in voltage control mode in order to minimize voltage drop when a further first group of wind turbines is connected to the further main transformer by closing a further first feeder switch. 
 
     
     
       11. Method according to  claim 1 , which involves, for coordinating the method, communication between at least a first controller and a second controller, the first controller controlling the feeder switches and the main breakers, the second controller controlling the wind turbine converters by sending them one of reference voltages and reference reactive power. 
     
     
       12. Method according to  claim 11 , wherein the second controller sends a message to the first controller indicating that a next feeder switch and/or a next breaker should be closed after having provided wind turbines with selected reference voltages. 
     
     
       13. Method according to  claim 11 , wherein the first controller or the second controller is a master controller and the other controller is a slave controller. 
     
     
       14. Method according to  claim 11 , wherein the first controller and the second controller operate according to a pre-agreed sequence and/or timing and/or delays of method steps. 
     
     
       15. Method according to  claim 11 , wherein the second controller determines reference voltages for the wind turbines so that inrush current from the utility grid is less than a threshold. 
     
     
       16. Method according to  claim 1 , performed for starting up after power outage to reduce inrush-current. 
     
     
       17. Method according to  claim 1 , wherein the first wind turbines comprise five to 20 wind turbines, wherein the first wind turbines are connected via separate switches to the first feeder switch sequentially. 
     
     
       18. Method according to  claim 1 , wherein at least one additional dynamic VAR system is connected to the point of interconnection and operated to stabilize voltage at the point of interconnection. 
     
     
       19. System for controlling coupling plural wind turbines to a utility grid (point of interconnection) after the wind turbines have been disconnected from the utility grid, the system comprising at least one controller adapted to
 control a main breaker arranged between the utility grid and a main transformer to close resulting in reactive current flowing from the utility grid to the main transformer; 
 control a first feeder switch arranged between the main transformer and a first group of wind turbines of the plural wind turbines to close resulting in reactive current flowing from the main transformer flowing to first wind turbine transformers of the first group of wind turbines; 
 select a first reference voltage based on an expected voltage drop that occurs when a second feeder switch arranged between the main transformer and a second group of wind turbines of the plural wind turbines is closed; 
 send a control signal to first converters of the first group of wind turbines to operate the first converters in a voltage control mode according to the first reference voltage; and 
 control the second feeder switch to close. 
 
     
     
       20. System according to  claim 19 , wherein the at least one controller comprises a first controller controlling the feeder switches and breakers and a second controller controlling the wind turbine converters.